Semiconductor device and method of manufacturing the same

ABSTRACT

A semiconductor device provided with a silicon carbide semiconductor substrate, and an ohmic metal layer joined to one surface of the silicon carbide semiconductor substrate in an ohmic contact and composed of a metal material whose silicide formation free energy and carbide formation free energy respectively take negative values. The ohmic metal layer is composed of, for example, a metal material such as molybdenum, titanium, chromium, manganese, zirconium, tantalum, or tungsten.

CROSS REFERENCE TO RELATED APPLICATION

This is a division of parent application Ser. No. 10/397,177, filed Mar.27, 2003 now U.S. Pat. No. 7,262,434. The entire disclosure of theparent application is incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor device such as aSchottky barrier diode using a silicon carbide semiconductor substrateand a method of manufacturing the same.

2. Description of Related Art

The configuration of a Schottky barrier diode using a silicon carbide(SiC) semiconductor substrate is as shown in FIG. 9 (e). One surface ofa SiC semiconductor substrate 100 is a silicon surface 110, and theother surface thereof is a carbon surface 120. A SiC epitaxial layer 101is formed on the side of the silicon surface 110.

A titanium (Ti) metal layer 30, for example, is formed on the siliconsurface 110, and a Schottky junction is formed in an interface betweenthe silicon surface 110 and the Ti metal layer 30. Further, an Alsurface electrode 50, for example, is formed on a surface of the Timetal layer 30 in order to ensure good adhesion to a metal wire composedof aluminum (Al) or the like, for example, for making connection to anexternal electrode.

On the other hand, a nickel (Ni) metal layer 20, for example, is formedon the carbon surface 120, and an ohmic junction is formed in aninterface between the carbon surface 120 and the Ni metal layer 20.Further, in order to satisfactorily connect the Schottky barrier diodeto an external substrate having copper (Cu) wiring, for example, asilver (Ag) reverse surface electrode 40, for example, is formed on asurface of the Ni metal layer 20.

In manufacturing the Schottky barrier diode, the Ni metal layer 20 isformed on the carbon surface 120 of the SiC semiconductor substrate 100having the epitaxial layer 101 on the side of the silicon surface 110(step T1 in FIG. 10). In order to form a good ohmic junction in theinterface between the carbon surface 120 and the Ni metal layer 20, theNi metal layer 20 is heat-treated at 1000° C. for twenty minutes (stepT2).

As shown in FIG. 9 (b), the Ti metal layer 30 is formed on the siliconsurface 110 of the SiC semiconductor substrate 100 (step T3 in FIG. 10).Thereafter, a resist 60 is applied to the surface of the Ti metal layer30 to pattern the Ti metal layer 30 (step T4), as shown in FIG. 9 (c),after which the Ti metal layer 30 is subjected to etching processing(step T5 in FIG. 10).

After the resist 60 is removed, the Ti metal layer 30 is heat-treated at400° C. for twenty minutes, as shown in FIG. 9 (d), in order to form agood Schottky junction to the SiC semiconductor substrate 100 (step T6in FIG. 10). Thereafter, the Al surface electrode 50 and the Ag reversesurface electrode 40 are respectively formed on the surfaces of the Timetal layer 30 and the Ni metal layer 20 (step T7), thereby forming theSchottky barrier diode shown in FIG. 9 (e).

As described in the foregoing, the Ni metal layer 20 and the Ti metallayer 30 are individually formed during manufacturing processes (stepsT1 and T3), and are individually heat-treated (steps T2 and T6).Therefore, the heat treatment must be carried out at least twice,thereby making it difficult to shorten the manufacturing processes.

Furthermore, the Ni metal layer 20 is used as an ohmic electrode. Unlessthe heat treatment is carried out at a high temperature of approximately1000° C., as shown in the step T2 in FIG. 10, therefore, a good ohmicjunction cannot be formed in the interface between the Ni metal layer 20and the carbon surface 120 of the SiC semiconductor substrate 100.Therefore, the operating characteristics of the Schottky barrier diodemay be adversely affected by the heat treatment during the manufacturingprocesses, resulting in poor manufacturing yields.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a semiconductor devicecapable of shortening manufacturing processes and a method ofmanufacturing the same.

Another object of the present invention is to provide a semiconductordevice whose operating characteristics may not be adversely affected bya heat treatment during manufacturing processes and a method ofmanufacturing the same.

A semiconductor device according to the present invention comprises asilicon carbide semiconductor substrate; and an ohmic metal layer joinedto one surface (which may be either a carbon surface or a siliconsurface, for example, the carbon surface) of the silicon carbidesemiconductor substrate in an ohmic contact and composed of a metalmaterial whose silicide formation free energy and carbide formation freeenergy respectively take negative values.

According to this configuration, the ohmic metal layer brought intoohmic contact with the silicon carbide semiconductor substrate is formedusing the metal material whose silicide formation free energy andcarbide formation free energy respectively take negative values. Theabove-mentioned metal material forms a good ohmic junction in aninterface to the silicon carbide semiconductor substrate by a heattreatment at a relatively low temperature (which is preferably atemperature of not less than the temperature of the silicon carbidesemiconductor substrate in a case where the semiconductor device isoperated, for example, 300° C. to 500° C.). Consequently, the adverseeffect on the operating characteristics by the heat treatment can beeliminated.

It is preferable that the ohmic metal layer is composed of a metalmaterial whose silicide formation free energy and carbide formation freeenergy at a temperature at the time of a heat treatment for joining theohmic metal layer to a surface of the silicon carbide semiconductorsubstrate in an ohmic contact respectively take negative values.

According to this configuration, the metal material which is a materialcomposing the ohmic metal layer can form a good ohmic junction becausethe silicide formation free energy and the carbide formation free energyof the metal material at the temperature at the time of the heattreatment respectively take negative values.

Examples of the above-mentioned metal material include at least onemetal material selected from a group consisting of molybdenum, titanium,chromium, manganese, zirconium, tantalum, and tungsten (which may be asimple substance or an alloy of two or more metal materials in thegroup).

It is preferable that the carrier concentration of a semiconductor layeron the side of the one surface of the silicon carbide semiconductorsubstrate is within a range of 10¹⁷ to 10²¹/cm³ (preferably 10¹⁹ to10²¹/cm³).

According to this configuration, the carrier concentration of thesemiconductor layer brought into contact with the ohmic metal layer issufficiently high, so that the ohmic metal layer is joined to thesurface of the silicon carbide semiconductor substrate with a lowresistance. Consequently, a good ohmic junction is formed.

There may be further provided a multi-layer metal structure having theohmic metal layer, and a metal layer composed of a metal materialdifferent from that composing the ohmic metal layer and formed on asurface of the ohmic metal layer.

If a metal material having good adhesion to a metal wire (a bondingwire) composed of aluminum, gold, or the like is used as theabove-mentioned other metal layer, external connection of the device canbe satisfactorily carried out.

When the semiconductor device is mounted on a package provided with anelectrode for external connection (an external electrode), for example,the external electrode and the ohmic metal layer can be satisfactorilyconnected to each other using a metal wire or a lead frame. When themetal wire connected between the external electrode and the ohmic metallayer is an aluminum wire, it is preferable that a metal layer composedof aluminum (Al), an aluminum-silicon alloy (Al/Si), or analuminum-silicon-copper alloy (Al—Si—Cu) is formed in the ohmic metallayer. When the external electrode and the ohmic metal layer aredirectly connected to each other, that is, the ohmic metal layer isdie-bonded to the lead frame, for example, it is preferable that a metallayer composed of silver (Au) or gold (Ag) is formed on a surface of theohmic metal layer. Consequently, the ohmic metal layer can be connectedto an external substrate composed of copper (Cu) wiring formed thereon,for example, with good adhesion.

Two or more metal layers may be provided on the surface of the ohmicmetal layer. When the ohmic metal layer is a titanium metal layer, forexample, a multi-layer metal structure in which a molybdenum metal layeris stacked on the titanium metal layer, and an aluminum metal layer, anAl—Si alloy metal layer, or the like is stacked on the molybdenum metallayer may be used. That is, adhesion of aluminum or an Al—Si alloy tothe titanium metal layer is not so good. However, a good adhesive stateis obtained in an interface between the metal layers by interposing themolybdenum metal layer therebetween, thereby making it possible tomanufacture a semiconductor device having high reliability.

The carrier concentration of a first semiconductor layer on the side ofthe one surface (for example, the carbon surface) of the silicon carbidesemiconductor substrate may be higher than the carrier concentration ofa second semiconductor layer on the side of the other surface (forexample, the silicon surface) of the silicon carbide semiconductorsubstrate. In this case, there may be further provided a Schottky metallayer joined to the other surface in a Schottky contact and composed ofthe same material as that composing the ohmic metal layer.

According to this configuration, a Schottky barrier diode composed of aSiC semiconductor is obtained. Metal layers having the same metalmaterial are respectively formed on both the surfaces of the siliconcarbide semiconductor substrate. Therefore, even if one of the metallayers is formed after the other metal layer is formed, neither of themetal layers is contaminated.

Furthermore, the metal material composing the ohmic metal layer and theSchottky metal layer is a metal material whose silicide formation freeenergy and carbide formation free energy respectively take negativevalues. Accordingly, an ohmic junction can be formed by a heat treatmentat a relatively low temperature. When the heat treatment at a relativelylow temperature is carried out after the ohmic metal layer and theSchottky metal layer are formed, therefore, the ohmic metal layerbrought into contact with the first semiconductor layer having arelatively high carrier concentration is satisfactorily joined to thefirst semiconductor layer in an ohmic contact, and a good Schottkyjunction is formed between the Schottky metal layer brought into contactwith the second semiconductor layer having a relatively low carrierconcentration and the second semiconductor layer. A Schottky barrierdiode having good characteristics can be manufactured by a single heattreatment at a relatively low temperature.

According to experiments conducted by the inventors of the presentapplication, it has become clear that the Schottky barrier diode havingthe above-mentioned configuration has good breakdown voltage. Further,metal layers composed of the same metal material are respectively formedon both the surfaces of the SiC semiconductor substrate. Accordingly,heat treatments on both the surfaces of the semiconductor substrate canbe carried out at one time, thereby making it possible to shorten themanufacturing processes.

It is preferable that the carrier concentration of the firstsemiconductor layer is within a range of 10¹⁷ to 10²¹/cm³ (preferably10¹⁹ to 10²¹/cm³). Further, the carrier concentration of the secondsemiconductor layer is preferably within a range of 10¹⁴ to 10¹⁶/cm³(preferably 10¹⁵ to 10¹⁶/cm³).

According to this configuration, the ohmic metal layer satisfactorilyjoined to the first semiconductor layer in an ohmic contact can beformed, and the Schottky metal layer satisfactorily joined to the secondsemiconductor layer in a Schottky contact can be formed.

One mode of a method of manufacturing a semiconductor device accordingto the present invention comprises a film forming step for forming ametal layer on one surface (which may be either a carbon surface or asilicon surface, for example, the carbon surface) of a silicon carbidesemiconductor substrate; and a heat-treating step for subjecting themetal layer to a heat treatment in a temperature range of 300° C. to500° C. after the film forming step, to form an ohmic junction betweenthe metal layer and the one surface of the silicon carbide semiconductorsubstrate. The film forming step comprises the step of forming the metallayer brought into contact with the silicon carbide semiconductorsubstrate using a metal material whose silicide formation free energyand carbide formation free energy at the temperature of the siliconcarbide semiconductor substrate in the heat-treating step respectivelytake negative values.

According to the present invention, the metal layer (ohmic metal layer)joined to the surface of the silicon carbide semiconductor substrate inan ohmic contact can be formed by the heat treatment in a temperaturerange of relatively low temperatures (which are preferably not less thanthe temperature of the silicon carbide semiconductor substrate in a casewhere the semiconductor device is operated, for example, 300° C. to 500°C.). Consequently, a silicon carbide semiconductor device having goodcharacteristics can be manufactured.

It is preferable that the film forming step comprises the step offorming the metal layer brought into contact with the silicon carbidesemiconductor substrate using at least one metal material selected froma group consisting of molybdenum, titanium, chromium, manganese,zirconium, tantalum, and tungsten.

The silicide formation free energy and the carbide formation free energyof the metal material respectively take negative values. Accordingly, ametal layer satisfactorily joined to the silicon carbide semiconductorsubstrate in an ohmic contact can be formed by the heat treatment at alow temperature.

Another mode of the present invention relates to a method ofmanufacturing a semiconductor device using a silicon carbidesemiconductor substrate in which the carrier concentration of a firstsemiconductor layer on the side of its one surface is higher than thecarrier concentration of a second semiconductor layer on the side of theother surface. This method comprises a first film forming step forforming a first metal layer brought into contact with the firstsemiconductor layer in the silicon carbide semiconductor substrate; asecond film forming step for forming a second metal layer brought intocontact with the second semiconductor layer in the silicon carbidesemiconductor substrate and composed of the same metal material as thatcomposing the first metal layer; a heat-treating step for simultaneouslyheat-treating the first metal layer and the second metal layer at apredetermined temperature after the first film forming step and thesecond film forming step, to form an ohmic junction between the firstmetal layer and the first semiconductor layer as well as to form aSchottky junction between the second metal layer and the secondsemiconductor layer. The first and second film forming steps comprisethe steps of respectively forming the first and second metal layersbrought into contact with the silicon carbide semiconductor substrateusing a metal material whose silicide formation free energy and carbideformation free energy at the temperature of the silicon carbidesemiconductor substrate in the heat-treating step respectively takenegative values.

When the second metal layer must be patterned, the first metal layer maybe formed after the second metal layer is formed.

According to this method, the Schottky barrier diode using the siliconcarbide semiconductor substrate can be manufactured. In this case, at asingle heat treatment, the first metal layer can be joined to thesilicon carbide semiconductor substrate in an ohmic contact in the firstsemiconductor layer having a relatively high carrier concentration, andthe second metal layer can be joined thereto in a Schottky contact onthe surface of the second semiconductor layer having a relatively lowcarrier concentration. Heat treatments for respectively forming theohmic metal layer and the Schottky metal layer can be simultaneouslycarried out, thereby making it possible to shorten the manufacturingprocesses.

It is preferable that the heat-treating step is the step of carrying outthe heat treatment within a temperature range of 300° C. to 500° C.

According to this method, the heat treatment is performed in a lowtemperature range of 300° C. to 500° C. Therefore, devicecharacteristics are not degraded by the heat treatment.

When the heat treatment is carried out at a temperature of not more than300° C., a good ohmic junction may not be formed. Further, a time periodrequired for the heat treatment is lengthened. Accordingly, a timeperiod required for the manufacturing processes is lengthened, therebyreducing the manufacturing efficiency. On the other hand, when the heattreatment is carried out at a temperature of not less than 500° C., aninterface to be a Schottky junction may undesirably become an ohmicjunction. If the heat treatment is carried out within a temperaturerange of 300° C. to 500° C., the time period required for themanufacturing processes may not be lengthened. Further, a good ohmicjunction and a Schottky junction can be formed.

It is preferable that the first and second film forming steps comprisethe steps of respectively forming the first and second metal layersbrought into contact with the silicon carbide semiconductor substrateusing at least one metal material selected from a group consisting ofmolybdenum, titanium, chromium, manganese, zirconium, tantalum, andtungsten.

The silicide formation free energy and the carbide formation free energyof the above-mentioned metal respectively take negative values. By theabove-mentioned heat treatment at a low temperature, therefore, thefirst metal layer can be satisfactorily joined to the firstsemiconductor layer in an ohmic contact, and the second metal layer canbe satisfactorily joined to the second semiconductor layer in a Schottkycontact.

The upper limit of a temperature at which the Schottky junction in aninterface between the silicon carbide semiconductor substrate and theabove-mentioned metal layer formed in contact therewith is changed to anohmic junction is approximately 500° C. Accordingly, it is preferablethat the heat-treating step is carried out within a temperature range of350° C. and 450° C. This eliminates the possibility that the operatingcharacteristics of the semiconductor device are affected by the heattreatment temperature during the manufacturing processes, thereby makingit possible to improve manufacturing yields.

It is preferable that the film forming step for forming on the firstmetal layer another metal layer composed of a metal material differentfrom the metal material composing the first metal layer is carried outafter the first film forming step, and the film forming step for formingon the second metal layer another metal layer composed of a metalmaterial different from the metal material composing the second metallayer is carried out after the second film forming step, and theheat-treating step is carried out after the other metal layers arerespectively formed on the first metal layer and the second metal layer.

Therefore, the multi-layer metal structure on the side of the firstmetal layer and the multi-layer metal structure on the side of thesecond metal layer can be together heat-treated. By the heat treatments,an ohmic junction is formed in the interface between the first metallayer and the silicon carbide semiconductor substrate, and a Schottkyjunction is formed in the interface between the second metal layer andthe surface of the silicon carbide semiconductor substrate. In addition,adhesion in the interface between the fist metal layer and the othermetal layer on the surface thereof and the interface between the secondmetal layer and the other metal layer on the surface thereof can be alsoenhanced.

The foregoing and other objects, features, aspects and advantages of thepresent invention will become more apparent from the following detaileddescription of the present invention when taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view schematically showing the configurationof a semiconductor device according to an embodiment of the presentinvention;

FIGS. 2 (a) to 2 (f) are diagrams schematically showing the steps ofmanufacturing a Schottky barrier diode according to an embodiment of thepresent invention;

FIG. 3 is a flow chart showing the steps of manufacturing a Schottkybarrier diode according to an embodiment of the present invention;

FIG. 4 is a diagram showing a silicide formation free energy;

FIG. 5 is a diagram showing a carbide formation free energy;

FIG. 6 is a diagram showing the forward voltage-forward currentcharacteristics of a Schottky barrier diode;

FIG. 7 is a diagram showing the on-resistance characteristics of aSchottky barrier diode;

FIG. 8 is a schematic cross-sectional view showing an example of theconfiguration of a Schottky barrier diode in which titanium metal layersare respectively formed on both surfaces of a silicon carbidesemiconductor substrate;

FIGS. 9 (a) to 9 (e) are diagram schematically showing the steps ofmanufacturing a conventional Schottky barrier diode; and

FIG. 10 is a flow chart showing the steps of manufacturing aconventional Schottky barrier diode.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 is a cross-sectional view schematically showing the configurationof a semiconductor device according to an embodiment of the presentinvention. The semiconductor device is a Schottky barrier diode, forexample. An example of a semiconductor substrate is an N-type SiCsemiconductor substrate (e.g., a 4H—SiC epiwafer) 1 having a surfaceorientation of {0001} and having an off angle of 8°. An edge termination15 is formed, in the SiC semiconductor substrate 1, by boronimplantation and annealing at a relatively low temperature(approximately 1000° C.).

One surface of the SiC semiconductor substrate 1 is a silicon surface11, and the other surface thereof is a carbon surface 12.

An N-type SiC semiconductor epitaxial layer 10 having a thickness ofapproximately 10 μm and having a carrier concentration of 4.0×10¹⁵/cm³to 8.0×10¹⁵/cm³ is formed on the silicon surface 11. Consequently, thecarrier concentration in the SiC semiconductor substrate 1 on the sideof the silicon surface 11 is relatively lower than that on the side ofthe carbon surface 12, and the carrier concentration in the SiCsemiconductor substrate 1 on the side of the carbon surface 12 isrelatively higher than that on the side of the silicon surface 11.Specifically, the carrier concentration on the side of the siliconsurface 11 is within a range of 10¹⁵ to 10¹⁶/cm³, and the carrierconcentration on the side of the carbon surface 12 is within a range of10¹⁸ to 10¹⁹/cm³.

A surface molybdenum metal layer 3 is formed on the silicon surface 11.The surface molybdenum metal layer 3 coats a predetermined region (aregion surrounded by the edge termination 15) of the silicon surface 11,and a Schottky junction is formed in an interface between the surfacemolybdenum metal layer 3 and the silicon surface 11 in contact with thesurface molybdenum metal layer 3.

A surface metal layer 5 formed of a metal other than a molybdenum metalis formed as a surface electrode on a surface of the molybdenum metallayer 3. The surface metal layer 5 is formed of a metal having goodadhesion to the surface molybdenum metal layer 3.

When an aluminum (Al) wire extending from an external electrode and thesurface molybdenum metal layer 3 are wire-bonded to each other, forexample, it is preferable that the surface metal layer 5 is formed ofaluminum (Al), an aluminum-silicon alloy (Al—Si), analuminum-silicon-copper alloy (Al—Si—Cu), or the like (a metal havinggood adhesion to aluminum). Consequently, the surface molybdenum metallayer 3 and the Al wire are electrically connected to each othersatisfactorily through the surface metal layer 5.

On the other hand, a reverse surface molybdenum metal layer 2 is formedon the carbon surface 12. The reverse surface molybdenum metal layer 2coats the overall carbon surface 12, and an ohmic junction is formed inan interface between the reverse surface molybdenum metal layer 2 andthe carbon surface 12 in contact with the reverse surface molybdenummetal layer 2.

A reverse surface metal layer 4 formed of a metal other than amolybdenum metal is formed as a reverse surface electrode on a surfaceof the reverse surface molybdenum metal layer 2. The reverse surfacemetal layer 4 is formed of a metal having good adhesion to the reversesurface molybdenum metal layer 2.

When copper (Cu) wiring formed on a wiring board and the reverse surfacemolybdenum metal layer 2 are connected to each other, for example, it ispreferable that the reverse surface metal layer 4 is formed of gold(Au), silver (Ag), or the like (a metal having good adhesion to copper).Consequently, the reverse surface molybdenum metal layer 2 and the Cuwiring are electrically connected to each other satisfactorily throughthe reverse surface metal layer 4.

FIGS. 2 (a) to 2 (f) are schematic cross-sectional views showing thesteps of manufacturing the Schottky barrier diode having theabove-mentioned configuration, and FIG. 3 is a flow chart showing thesteps of manufacturing the Schottky barrier diode.

As shown in FIG. 2 (a), a silicon surface 11 of a SiC semiconductorsubstrate 1 having an epitaxial layer 10 formed on the silicon surface11 (in which an edge termination 15 has already been formed) issubjected to acid cleaning such as RCA cleaning, and is then subjectedto dilute hydrofluoric acid cleaning, to form a surface molybdenum metallayer 3 (step Si in FIG. 3). The surface molybdenum metal layer 3 isformed using a sputtering method carried out by causing argon ions tocollide with a molybdenum target. The degree of vacuum in a sputterchamber in which argon gas has not been introduced yet is 10⁻³ to 10⁻⁴Pa, for example.

When the film formation processing of the surface molybdenum metal layer3 is terminated, a metal layer composed of a metal other than molybdenumis formed on a surface of the surface molybdenum metal layer 3 using asputtering method (step S2), as shown in FIG. 2 (b), to form a surfacemetal layer 5. The surface metal layer 5 is formed by so-calledcontinuous sputtering processing performed in a processing chamber wherethe surface molybdenum metal layer 3 is formed (step S1) by replacingthe target with the inside of the chamber kept in a high vacuum statewithout being brought into an atmospheric state. At this time, I-Vcharacteristics must be degraded (a forward rise voltage V_(F) isincreased, for example) if the inside of the chamber is brought into anatmospheric state once without performing continuous sputtering.

In order to pattern the surface molybdenum metal layer 3 and the surfacemetal layer 5, as shown in FIG. 2 (c), photolithographic processing isthen performed by application of a resist 6 and exposure processing(step 3 in FIG. 3). After the photolithographic processing isterminated, the surface molybdenum metal layer 3 and the surface metallayer 5 are patterned in a region surrounded by the edge termination 15by being etched together using the resist 6 as a mask (step S4), asshown in FIG. 2 (d). Thereafter, the resist 6 is removed.

Thereafter, after the dilute hydrofluoric acid cleaning, a reversesurface molybdenum metal layer 2 is formed on the carbon surface 12 ofthe SiC semiconductor substrate 1 by a sputtering method (step S5), asshown in FIG. 2 (e). At this time, it is preferable that before aforward voltage at the time of the sputtering is applied, reversesputtering for applying a reverse voltage is performed such that aninterface between the reverse surface molybdenum metal layer 2 and thecarbon surface 12 form a good ohmic junction. Consequently, the argonions are irradiated onto the carbon surface 12 of the SiC semiconductorsubstrate 1, thereby removing an undesirable oxide film on the carbonsurface 12.

When the film formation processing of the reverse surface molybdenummetal layer 2 is terminated, a metal layer composed of a metal otherthan molybdenum is formed on a surface of the reverse surface molybdenummetal layer 2 using a sputtering method (step S6), as shown in FIG. 2(f), to form a reverse surface metal layer 4. The reverse surface metallayer 4 is formed by so-called continuous sputtering processingperformed in a processing chamber where the reverse surface molybdenummetal layer 2 is formed (step S5) by replacing the target with theinside of the chamber kept in a high vacuum state without being broughtinto an atmospheric state. At this time, I-V characteristics must bedegraded (a forward rise voltage V_(F) is increased, for example) if theinside of the chamber is brought into an atmospheric state once withoutperforming continuous sputtering.

In such a way, the surface molybdenum metal layer 3 and the surfacemetal layer 5 are stacked on the silicon surface 11 of the SiCsemiconductor substrate 1, and the reverse surface molybdenum metallayer 2 and the reverse surface metal layer 4 are stacked on the carbonsurface 12 of the SiC semiconductor substrate 1, after which a heattreatment is performed at 400° C. for twenty minutes, for example (stepS7). By a single heat treatment, a good Schottky junction is formed inthe interface between the silicon surface 11 and the surface molybdenummetal layer 3 formed on the silicon surface 11, and adhesion in theinterface between the surface molybdenum metal layer 3 and the surfacemetal layer 5 is strengthened. At the same time, a good ohmic junctionis formed in the interface between the carbon surface 12 and the reversesurface molybdenum metal layer 2 formed on the carbon surface 12, andadhesion in the interface between the reverse surface molybdenum metallayer 2 and the reverse surface metal layer 4 is strengthened.

That is, a metal joined to the silicon surface 11 in a Schottky contactand a metal joined to the carbon surface 12 in an ohmic contact are bothmade of molybdenum, thereby making it possible to together heat-treatthe two molybdenum metal layers 2 and 3 and the two metal layers 4 and5. Consequently, the manufacturing processes for the Schottky barrierdiode can be significantly shortened.

The heat treatment temperature is within a temperature range of 300° C.and 500° C. (e.g., 350° C. to 450° C.). Consequently, a temperature muchlower than a heat treatment temperature of 1000° C. conventionallyrequired when an Ni metal layer has been used is sufficient to carry outthe heat treatment. Therefore, the operating characteristics of theSchottky barrier diode may not be adversely affected by the heattreatment temperature during the manufacturing processes, thereby makingit possible to improve manufacturing yields.

If the heat treatment is carried out at a temperature of not more than300° C., for example, a good ohmic junction cannot be formed in theinterface between the carbon surface 12 and the reverse surfacemolybdenum metal layer 2 formed in contact with the carbon surface 12,and a time period required for the manufacturing processes is furtherlengthened, thereby reducing the manufacturing efficiency. If the heattreatment is carried out at a temperature of not less than 500° C., aSchottky junction in the interface between the silicon surface 11 andthe surface molybdenum metal layer 3 formed in contact with the siliconsurface 11 is changed to an ohmic junction. If the heat treatment iscarried out within a temperature range of 300° C. to 500° C., a timeperiod required for the manufacturing processes may not be lengthened,and it is also possible to form a good ohmic junction in the interfacebetween the carbon surface 12 and the reverse surface molybdenum metallayer 2 formed in contact with the carbon surface 12. At the same time,a good Schottky junction can be formed in the interface between thesilicon surface 11 and the surface molybdenum metal layer 3 formed incontact with the silicon surface 11.

The upper limit of a temperature at which the Schottky junction in theinterface between the silicon surface 11 and the surface molybdenummetal layer 3 formed in contact with the silicon surface 11 is changedto an ohmic junction is 500° C. Accordingly, it is preferable that theheat-treating step is carried out within a temperature range of 350° C.and 450° C.

The Schottky barrier diode thus manufactured has good operatingcharacteristics with a reverse breakdown voltage of 600V to 1000V, and aforward rise voltage of 1.3 V/1 A. Consequently, a Schottky barrierdiode which sufficiently stands practical use can be manufacturedalthough the manufacturing processes are shortened by using themolybdenum metal layers 2 and 3 in common for both surfaces of the SiCsemiconductor substrate 1.

A Schottky barrier diode having substantially the same characteristicscan be also obtained by using titanium (Ti), chromium (Cr), manganese(Mn), zirconium (Zr), tantalum (Ta), or tungsten (W), or an alloy of twoor more metals selected therefrom in common as a material for the metallayers 2 and 3 for forming an ohmic junction and a Schottky junction inplace of molybdenum (Mo). Although the same is true for the case ofmolybdenum, the above-mentioned metals react with SiC in conformity withthe following reaction formula:SiC+M→M_(x)Si_(y)+M_(m)C_(n)

where M indicates a metal, and x, y, m, and n are natural numbers.

That is, the above-mentioned metals react with both Si and C in SiC. Asa result, an ohmic junction is easy to form in an interface between themetal and SiC.

Contrary to this, metals such as iron (Fe), nickel (Ni), copper (Cu),and lead (Pb) react with only C in SiC, as expressed by the followingreaction formula:SiC+M→M_(x)Si_(y)+C

Furthermore, aluminum (Al) or the like reacts with only Si in SiC, asexpressed by the following reaction formula:SiC+M→M_(m)C_(n)+Si

That is, the metals such as iron, nickel, copper, lead, and aluminumreact with either C or Si in SiC. As a result, an ohmic junction isdifficult to form in an interface between the metal and SiC.

As to molybdenum, titanium, chromium, zirconium, tantalum, and tungsten,the free energy of formation (Gibbs free energy of formation ΔG) of thesilicide of each of the metals is less than zero (a negative value), andthe free energy of formation of the carbide of the metal is also lessthan zero. The smaller the free energy of formation is, the more easilya chemical reaction for formation occurs. When the free energy offormation takes a negative value, the chemical reaction spontaneouslyproceeds. On the other hand, when the free energy of formation takes apositive value, energy must be applied from the exterior in order toadvance a chemical reaction for forming a product of the metal.

Therefore, it can be said that the metals, in the above-mentioned group,whose silicide formation free energy and carbide formation free energyrespectively take negative values are materials which easily react withboth Si and C in SiC to form a good ohmic junction.

FIG. 4 is a diagram showing the respective free energies of formation ofthe silicides of molybdenum, titanium, and nickel. As can be seen fromFIG. 4, all the free energies of formation of the silicides ofmolybdenum, titanium, and nickel respectively take negative values in atemperature range of 400° C. to 1000° C.

FIG. 5 is a diagram showing the respective free energies of formation ofthe carbides of molybdenum, titanium, nickel, and aluminum. As can beseen from FIG. 5, the free energies of formation of the carbides ofaluminum, molybdenum, and titanium respectively take negative values ina temperature range of 400° C. to 1000° C. Contrary to this, the freeenergy of formation of Ni₃C which is the carbide of nickel takes apositive value in a temperature range of 400° C. to 1000° C. From theforegoing, it is understood that a good ohmic junction is not easilyobtained even if a nickel metal film is formed on a surface of a SiCsemiconductor substrate.

Aluminum, which is not illustrated in FIG. 4, does not form a silicideby reacting with Si. Therefore, the silicide formation free energy takesa value which is too large to be illustrated in FIG. 4. If an aluminummetal layer is formed on the surface of the SiC semiconductor substrate,therefore, no good ohmic junction can be obtained.

In order to form a good ohmic junction on the surface of the SiCsemiconductor substrate, it is preferable that both the carbide andsilicide formation free energies are lower. Accordingly, a better ohmicjunction can be formed in a case where titanium is used, as comparedwith that in a case where molybdenum is used.

FIGS. 4 and 5, described above, are produced on the basis of thedescription of a literature entitled by Shigeyuki Somiya and KichizoInomata “New Material Series Silicon Carbide Ceramics”, SemiconductorDevice R&D Division, ROHM CO., LTD., published by Uchida Rokakuho Pub.,15 Sep. 1988, p. 177-182. According to the description of theliterature, the respective free energies of formation of the silicideand the carbide of zirconium are lower than those of titanium.Consequently, it is considered that a better ohmic junction can befurther formed in a case where zirconium is used, as compared with thatin a case where titanium is used.

The inventors of the present application have observed the respectiveinterfaces between the silicon surface and the carbon surface and themetal layers 2 and 3 using a transmission electron microscope (TEM) withrespect to the Schottky barrier diode manufactured by respectivelyjoining the molybdenum metal layers 2 and 3 to both the surfaces of theSIC semiconductor substrate 1.

As a result, it is confirmed that a region having a thickness ofapproximately 30 Å which is considered to a paracrystal having aperiodic crystalline structure maintained on its SiC crystal interfaceis formed on the carbon surface 12. That is, it is confirmed that amolybdenum metal is sufficiently diffused into the surface of the SiCsemiconductor substrate 1, so that a good ohmic junction is obtained.

On the other hand, on the silicon surface 11 side, it is confirmed thatan amorphous alloy region having a thickness of approximately 10 Å isformed in the interface between the molybdenum metal layer 3 and the SiCsemiconductor substrate 1. Consequently, it can be said that a goodSchottky junction is formed on the silicon surface 11.

Furthermore, the inventors of the present application have analyzed anamorphous alloy region in the interface between the silicon surface 11and the molybdenum metal layer 3 using energy disperse X-ray (EDX). Theresults of the analysis prove that the amorphous alloy region containsan oxygen atom.

Similarly, the inventors of the present application have analyzed theinterface between the carbon surface 12 and the molybdenum metal layer 2by EDX. The results of the analysis prove that an oxygen atom exists ina SiC/Mo diffusion region where a paracrystal seems to be formed.

It is considered that the reason why an oxygen atom also exists in anohmic junction on the carbon surface 12 of the SiC semiconductorsubstrate 1 is that cleaning processing or the like is performed in air.However, it is still further confirmed that the Schottky barrier diodeaccording to the above-mentioned embodiment still has goodcharacteristics.

Specifically, FIG. 6 illustrates the relationship of a forward currentI_(F) to a forward voltage V_(F) in the above-mentioned Schottky barrierdiode. As compared with a conventional SiC Schottky barrier diodeindicated by a curve L1 (Ni is used for an ohmic electrode, and Ti isused for a Schottky electrode), the better rise characteristics of theforward current are obtained, as indicated by a curve L2, in the SiCSchottky barrier diode according to the above-mentioned embodiment inwhich the molybdenum metal layers 2 and 3 are applied in common to thecarbon surface 12 and the silicon surface 11. That is, the forward risevoltage V_(F) can be reduced.

Furthermore, in a case where a metal layer composed of atitanium-molybdenum alloy (Ti—Mo) is applied to the carbon surface 12and the silicon surface 11 of the SiC semiconductor substrate 1, thebetter forward rise characteristics are obtained, as compared with thatin the case where the molybdenum metal layer is used, as indicated bythe curve L3.

A curve L0 indicates the characteristics of the Schottky barrier diodeusing the Si semiconductor substrate.

FIG. 7 illustrates the on-resistance characteristics of the SiC Schottkybarrier diode. A curve L11 indicates the limit value of an on-resistancein the silicon semiconductor device, and a curve L12 indicates the limitvalue of an on-resistance in the 4H—SiC semiconductor device.

The Schottky barrier diode according to the present embodiment in whichthe molybdenum metal layer is formed on the carbon surface and thesilicon surface of the SiC semiconductor substrate exhibits asignificantly low on-resistance, as indicated by point P1. If titaniummetal layers are respectively applied to both surfaces of the SiCsemiconductor substrate, it is expected that on-resistancecharacteristics in the vicinity of region A are obtained.

The on-resistance characteristics of a conventional product (using Nifor an ohmic electrode and using Ti for a Schottky electrode) areindicated by point P10.

FIG. 8 is a schematic cross-sectional view showing a structural examplein a case where titanium metal layers are respectively formed on bothsurfaces of a SiC semiconductor substrate, to produce a Schottky barrierdiode. In the structural example, molybdenum metal layers arerespectively formed on surfaces of the titanium metal layers on bothsurfaces of the SiC semiconductor substrate 1, and an Al—Si alloy layersuperior in adhesion to an aluminum wire is formed on the molybdenummetal layer on the side of a Schottky electrode. Further, a metal layercomposed of gold (Au), silver (Ag), or a gold-silver alloy (Au—Ag) isformed on the molybdenum metal layer on the side of an ohmic electrode.Accordingly, good die-bonding to a lead frame or the like is possible.

Although description was made of one embodiment of the presentinvention, the present invention can be embodied in other ways. Althoughin the above-mentioned embodiment, description has been made of anexample in which the metal layers composed of molybdenum or the like arerespectively formed on both the surfaces of the SiC semiconductorsubstrate to configure the Schottky barrier diode, the present inventionis applicable to an arbitrary SiC semiconductor device in which anelectrode (a source electrode, a drain electrode, etc.) joined to asurface of the SiC semiconductor substrate in an ohmic contact must beprovided, for example, a Junction field effect transistor using a SiCsemiconductor substrate, a MOS field effect transistor (MOSFET), and anintegrated circuit device.

Although in the above-mentioned embodiment, description has been made ofan example using the N-type SiC semiconductor substrate, a P-type SiCsemiconductor substrate may be used, in which case a SiC semiconductordevice such as a Schottky barrier diode can be also manufactured.

Although in the above-mentioned embodiment, description has been made ofan example in which an electrode joined to the silicon surface of theSiC semiconductor substrate in a Schottky contact is formed, and anelectrode joined to the carbon surface in an ohmic contact is formed, aSchottky barrier diode may be manufactured by making the carrierconcentration of a semiconductor layer on the side of a silicon surfacehigher than the carrier concentration of a semiconductor layer on theside of a carbon surface to form an ohmic electrode on the siliconsurface and form a Schottky electrode on the carbon surface.

Although the present invention has been described and illustrated indetail, it is clearly understood that the same is by way of illustrationand example only and is not to be taken by way of limitation, the spiritand scope of the present invention being limited only by the terms ofthe appended claims.

This application corresponds to Japanese Patent Application Serial Nos.2002-92530 and 2002-335414 respectively filed with the Japanese PatentOffice on Mar. 28, 2002 and Nov. 19, 2002, the disclosures of which areincorporated herein by reference.

1. A method of manufacturing a semiconductor device, comprising: a firstfilm forming step for forming a metal layer on a first surface of asilicon carbide semiconductor substrate; a second film forming step forforming a second metal layer on a second surface of the silicon carbidesemiconductor substrate opposing the first surface, the second metallayer composed of a same metal material as that composing the firstmetal layer, the metal material comprising molybdenum; and aheat-treating step for simultaneously subjecting the first and secondmetal layers to a heat treatment in a temperature range of 350° C. to450° C. after the first and second film forming steps, to form an ohmicjunction between the first metal layer and the first surface of thesilicon carbide semiconductor substrate as well as to form a Schottkyjunction between the second metal layer and the second surface of thesilicon carbide semiconductor substrate, the first and second filmforming steps including the steps of respectively forming the first andsecond metal layers brought into contact with the silicon carbidesemiconductor substrate using a metal material whose silicide formationfree energy and carbide formation free energy at the temperature of thesilicon carbide semiconductor substrate in the heat-treating steprespectively take negative values.
 2. The method according to claim 1,wherein the first and second film forming steps include the steps ofrespectively forming the first and second metal layers brought intocontact with the silicon carbide semiconductor substrate, in which themetal material further comprises a metal selected from a groupconsisting of titanium, chromium, manganese, zirconium, tantalum, andtungsten.
 3. A method of manufacturing a semiconductor device using asilicon carbide semiconductor substrate in which carrier concentrationof a first semiconductor layer on a side of a first surface thereof ishigher than carrier concentration of a second semiconductor layer on aside of a second surface thereof, comprising: a first film forming stepfor forming a first metal layer brought into contact with the firstsemiconductor layer in the silicon carbide semiconductor substrate; asecond film forming step for forming a second metal layer brought intocontact with the second semiconductor layer in the silicon carbidesemiconductor substrate and composed of a same metal material as thatcomposing the first metal layer, the metal material comprisingmolybdenum; and a heat-treating step for simultaneously heat-treatingthe first metal layer and the second metal layer at a predeterminedtemperature in a temperature range of 350° C. to 450° C. after the firstfilm forming step and the second film forming step, to form an ohmicjunction between the first metal layer and the first semiconductor layeras well as to form a Schottky junction between the second metal layerand the second semiconductor layer, the first and second film formingsteps including the steps of respectively forming the first and secondmetal layers brought into contact with the silicon carbide semiconductorsubstrate using a metal material whose silicide formation free energyand carbide formation free energy at the temperature of the siliconcarbide semiconductor substrate in the heat-treating step respectivelytake negative values.
 4. The method according to claim 3, wherein thefirst and second film forming steps include the steps of respectivelyforming the first and second metal layers brought into contact with thesilicon carbide semiconductor substrate, in which the metal materialfurther comprises a metal selected from a group consisting of titanium,chromium, manganese, zirconium, tantalum, and tungsten.
 5. A method ofmanufacturing a semiconductor device using a silicon carbidesemiconductor substrate having a silicon surface and a carbon surface,comprising: a carbon surface side metal layer forming step of forming acarbon surface side metal layer brought into contact with the carbonsurface of the silicon carbide semiconductor substrate, the carbonsurface side metal layer being composed of either molybdenum or an alloyof molybdenum and at least one of the metals selected from a group oftitanium, chromium, manganese, zirconium, tantalum and tungsten; asilicon surface side metal layer forming step of forming a siliconsurface side metal layer brought into contact with the silicon surfaceof the silicon carbide semiconductor substrate, the silicon surface sidemetal being composed of a same metal material as the carbon surface sidemetal layer; and a heat-treating step of simultaneously heat-treatingthe carbon surface side metal layer and the silicon surface side metallayer at a predetermined temperature in a temperature range of 350° C.to 450° C. after the carbon surface side metal layer forming step andthe silicon surface side metal layer forming step, to form an ohmicjunction between the carbon surface side metal layer and the carbonsurface of the silicon carbide semiconductor substrate as well as toform a Schottky junction between the silicon surface side metal layerand the silicon surface of the silicon carbide substrate.
 6. The methodaccording to claim 5, wherein the silicon carbide semiconductorsubstrate has a first semiconductor layer on the carbon surface side anda second semiconductor layer on the silicon surface side, the firstsemiconductor layer having a higher carrier concentration than that ofthe second semiconductor layer.